Clamping mandrels are usually used to clamp rolls of printing substrate webs, for example newsprint, by axially shifting two mandrels from opposed sides into support sleeves of the respective roll. The end portions of these mandrels taper towards a usually blunt tip to facilitate insertion.
Clamping mandrels of the type to which the present invention relates are well known, and one such mandrel is described, for example, in U.S. Pat. No. 5,123,604 Hammer et al. The side walls of the roll changers have carrier arms, rotatable about an axis. The carrier arms are spaced apart or can be shifted to accept the width of the roll of substrate web to be accomodated, and the holding mandrels are then shifted axially to fit into the inner roll-up sleeves of the respective substrate web rolls. The mandrels at the opposite sides of the roll can be identical. A roll of web material can then be clamped between such essentially identical mandrels. The mandrels, or at least one of them, are coupled to a drive system, usually located at the respective end of the carrier arms. The drive system permits rotation of the mandrels about their axes, as well as axial movement of the mandrels when a new roll is to be inserted, or an old roll, or the sleeve of an old roll is to be removed.
Substrate webs, and particularly newsprint rolls, are heavy, weighing several tons. These rolls are usually supplied to the roll changers by an automatic transport system, or delivered from trucks. A lifting hoist, which may already be installed on a truck, raises the rolls to a position where the mandrels can be inserted into the central sleeves at a loading position. This raising is difficult since the very heavy rolls must be aligned approximately axially with respect to the clamping mandrels. Any positioning errors, or positioning tolerances, as well as deviations of the geometrical configuration of the web roll from an ideal or standard size may result in misalignment between the roll and the clamping mandrel. Further, the cross section of the central sleeve about which the substrate web is wound may not be entirely precisely circular, or the sleeve may be somewhat eccentric with respect to the substrate web wound thereon. If the transport and raising system is automatically operated, deviations of the loading position from the axial alignment of the clamping mandrel are practically unavoidable.
Usually, the clamping mandrels have tapered ends, which are somewhat blunt, in order to minimize danger of damage to the sleeve, and facilitate insertion of the mandrel even though there may be misalignment upon placement of the roll in the roll changer apparatus, and prior to insertion of the clamping mandrels.
If the positioning error between the substrate web roll and the clamping mandrel is high, and, particularly if the loading system is controlled by an automatic sequencing apparatus, the situation may arise that the end portions of the clamping mandrel engage against the end surfaces of the sleeve and, upon a serious error, even against the end surfaces of the substrate material wound in a roll. If the end portion of the clamping mandrel engages the sleeve, damage to the sleeve may result since the clamping mandrels are pressed into the sleeve with substantial force. This damage may be severe enough to inhibit reception of the roll on the carrier arms; upon excessive misalignment, damage to the web may be severe enough to prevent use of the web in a web printing machine, since uniform pull-off and unrolling of the substrate web from the roll can no longer be ensured.
It has been proposed to provide a light gate to measure the position of the roll, and its sleeve, prior to engagement of the mandrel with the sleeve. This, however, is an indirect way of measuring, requires an additional measuring apparatus and a measuring position, and an additional measuring location coupled to the control for the automatic transport system or to the lifting arrangement, which lifting arrangement must position the roll. An additional measuring system, further, may introduce additional errors due to the additional measuring itself. Experiments have shown that light gates to measure the relative position of the sleeve and the mandrel and determine alignment of the sleeve with respect to the mandrel are not accurate enough since the light beam which is usually used cannot be focussed accurately enough if rolls of widely different varying standard diameters and widths have to be accepted by the roll changer. This is particularly annoying when the widths of the rolls are subject to wide variations.